Whatever cable is selected it must be capable of continually carrying the currents required by the loads. This is referred to as a cable’s full thermal current rating. Incorrect specification of cables can result in prolonged operation beyond its designed temperature rating, which can cause premature failure, or at worst, a potential fire risk.
The current calculating capability of a cable is determined by calculating the maximum current (Iz) required and the ac supply rating. This calculation is based upon several factors including the design current (Ib) of the circuit and the maximum input rating of the UPS that is to be installed, the nominal rating of each of the protective devices (In) and the cable rating (It). Cable rating is the value of current for a particular conductor located within a specific environment.
The current carrying capability of non-armoured, multi-core cables (with thermoplastic PVC or thermosetting insulation) can be affected by location. Current carrying capability reduces under load due to reduced heat dissipation. For example, a three or four-core 10mm cable enclosed within an insulated wall, the maximum current carrying capability is reduced to 39A. In free air, current carrying capability would increase to 60A because heat from the cable is more easily dissipated.
It is also important to calculate voltage drop based on the length of the cable and quantity of current passing through it at full operating load. The greater the load or distance, the greater the voltage drop. Wiring regulation standards (in the UK BS EN 7671 IEE Wiring Regulations 17th Edition) throughout the world specify requirements that must be met for acceptable levels of voltage drop and so forth.
Other factors to consider when sizing and installing cables in uninterruptible power supply installations are Earth fault loop impedance; operating current (Ia); ambient temperature; thermal insulation (Ci) and grouping (Cg).
Earth Fault Loop Impedance
As part of the electrical installation of an uninterruptible power supply it is important to calculate this value as it must satisfy regulations. An acceptable value will also ensure the necessary fault current will be generated should a fault condition or short-circuit occur. Once the level of fault current has been calculated, it can be checked against the time curve of the protective device to ensure it will trip appropriately if necessary.
Operating Current (Ia)
Operating current is the current required to trip the chosen protective device within the required operating time during a fault condition.
Ambient Temperature (Ca)
As already highlighted, ambient temperature around the installed cables must be considered to ensure, during operation, that they do not overheat and can dissipate heat effectively. This is especially important where there are high ambient temperatures and/or direct heat sources such as radiators or heaters.
Thermal Insulation (Ci)
When cables are installed in an insulated location, that insulation will restrict the cable from effectively dissipating heat and that should be borne in mind in any electrical installation.
Grouping (Cg)
Heat dissipation will also be compromised when several cables are grouped together within one location. It is inevitable, particularly in new or purpose-built installations, that there is a necessity for this to happen. As long as all the above are taken into consideration and accurate calculations made, grouping can be put into effect in many installations.
This article was compiled using information available in The Power Protection Guide – the design, installation and operation of uninterruptible power supplies (ISBN: 9 780955 442803). By Robin Koffler and Jason Yates of Riello UPS.
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